網(wǎng)絡(luò)通信_ch6_數(shù)據(jù)通信接口

1、Bilingual CourseNetwork Communications網(wǎng)絡(luò)通信網(wǎng)絡(luò)通信(For Master Students in the Department of Electronic Engineering)Chapter 6 The Data Communications Interface網(wǎng)絡(luò)通信2Outline of Chapter 6The transmission of a stream of bits from one device to another across a transmission link involves a great deal of coope

2、ration and agreement. One of the most fundamental requirements is synchronization. The receiver must know the rate at which bits are being received so that it can sample the line at require intervals to determine the value of each received bit.In asynchronous transmission, each character of data is

3、treated independently. For large blocks, asynchronous transmission is used. Each block of data is formatted as a frame that includes a starting and an ending.For a device to transmit across a medium, it must be attached through some sort of interface. It defines not only the electrical characteristi

4、cs of the signal but also the physical means of attachment and the procedures for sending and receiving bits of data.網(wǎng)絡(luò)通信36-1 Asynchronous and Synchronous Transmission 說明說明 本節(jié)主要討論信號的串行傳輸 假定每個(gè)信號元素表示一個(gè)bit 兩個(gè)主要問題兩個(gè)主要問題 噪聲等傳輸損傷對接收數(shù)據(jù)的影響，可能導(dǎo)致出錯(cuò); 時(shí)序問題，若發(fā)/收不精確一致，也可能出錯(cuò)，且這種時(shí)間上的不一致會(huì)積累.網(wǎng)絡(luò)通信41.異步和同步傳輸異步和同步傳輸（1）異

5、步傳輸）異步傳輸1)概念概念 In asynchronous transmission, each character of data is treated independently. Each character begins with a start bit that alerts the receiver that a character is arriving. The receiver samples each bit in the character and then looks for the beginning of the next character. 2)方法方法 Th

6、e strategy with this scheme is to avoid the timing problem by not sending long, uninterrupted streams of bits. Instead, data are transmitted one character at a time, where each character is five to eight bits in length. Timing or synchronization must only be maintained within each character; the rec

7、eiver has the opportunity to resynchronize at the beginning of each new character.3）工作過程工作過程 （Please see Figure 6.1 in Page 176, E6, Next slide）4)優(yōu)缺點(diǎn)優(yōu)缺點(diǎn)優(yōu)點(diǎn)：僅需一條信道，且傳輸距離較遠(yuǎn)；缺點(diǎn)：額外開銷較大（約20%）。網(wǎng)絡(luò)通信5網(wǎng)絡(luò)通信6Description on Figure 6.1不傳送字符時(shí)，發(fā)/收之間的線路處于“空閑”狀態(tài)；字符的傳送從“起始位（start bit）”開始（二進(jìn)制0），其后跟著組成字符的58bit數(shù)據(jù)；數(shù)據(jù)字符的傳送

8、從最低位開始；在數(shù)據(jù)bit之后，通常跟有一個(gè)奇偶校驗(yàn)位（odd or even parity bit），校驗(yàn)位的值由發(fā)送器設(shè)置。其作用是使整個(gè)字符（包括校驗(yàn)位本身）的二進(jìn)制1的總數(shù)為偶數(shù)（稱為偶校驗(yàn)）或奇數(shù)（稱為奇校驗(yàn)）；最后一個(gè)元素是“停止位（stop element）”。發(fā)送器不停地發(fā)送停止位，直到出現(xiàn)下一個(gè)起始位。網(wǎng)絡(luò)通信7Effect of Timing ErrorThe timing requirements for this scheme are modest（適中的）. For example, IRA (International Reference Alphabet) cha

9、racter are typically sent as 8-bit units, including the parity bit. If the receiver is 5 percent slower or faster than the transmitter, the sampling of the eight character bit will be displaced（移動(dòng)）45 percent and still be correctly sampled. Figure 6.1(c) shows the effects of a timing error of suffici

10、ent magnitude to cause an error in reception. In this example we assume a data rate of 10,000 bits per second (10kbps); therefore, each bit is 0.1 ms, or 100us, duration. Assume that the receiver is fast by 6 percent, or 6us per bit time. Thus, the receiver samples the incoming character every 94us.

11、 As can be seen, the last sample is erroneous. 網(wǎng)絡(luò)通信8 兩種錯(cuò)誤：兩種錯(cuò)誤： The last sampled bit is incorrectly received. The bit count may now be out of alignment. If bit 7 is a 1 and bit 8 is a 0, bit 8 could be mistaken for a start bit. This condition is termed a framing error（組幀錯(cuò)誤）, as the character plus st

12、art bit and stop element are sometimes referred to as a frame. A frame error can also occur if some noise condition causes the false appearance of a start bit during the idle state網(wǎng)絡(luò)通信9（2）同步傳輸）同步傳輸1）概念）概念 With synchronous transmission, a block of bits is transmitted in a steady stream without start

13、and stop codes. The block may be many bits in length. To prevent timing drift between transmitter and receiver, their clocks must somehow（某種方式）be synchronized. One possibility is to provide a separate clock line between transmitter and receiver. One side (transmitter or receiver) pulses the line reg

14、ularly with one short pulse per bit time. The other side uses these regular pulses as a clock. This technique works well over short distances as the data signal, but over long distances the clock pulses are subject to the same impairment （損傷）as the data signal, and timing errors can occur. 網(wǎng)絡(luò)通信10The

15、 other alternative is to embed the clocking information in the signal. For digital signals, this can be accomplished with Manchester or Differential Manchester encoding. For analog signals, a number of techniques can be used; for example, the carrier frequency itself can be used to synchronize the r

16、eceiver based on the phase of the carrier. 每個(gè)數(shù)據(jù)塊以一個(gè)“前同步碼（preamble）”開始，用一個(gè)“后同步”碼（postamble）”結(jié)束。此外，數(shù)據(jù)中的控制信息中也有一部分是起同步作用的。方法（如何讓接收器判斷數(shù)據(jù)塊的開始和結(jié)束）網(wǎng)絡(luò)通信11嵌入式時(shí)鐘編碼舉例嵌入式時(shí)鐘編碼舉例網(wǎng)絡(luò)通信123）幀格式）幀格式 特點(diǎn)：特點(diǎn)： - 以前同步碼開始，長度為8bit； - 后同步碼使用相同的標(biāo)志； - 前同步碼后緊跟一些控制字段； - 再以后是數(shù)據(jù)字段； - 數(shù)據(jù)字段后，還有控制字段。 - 例：對于1000個(gè)字符的數(shù)據(jù)塊，數(shù)據(jù)量為1000*8=8000b

17、it，若每個(gè)幀包含48bit的額外開銷，則48/8000=0.6%. - 優(yōu)點(diǎn)：額外開銷?。蝗秉c(diǎn)：需要獨(dú)立的時(shí)鐘，或編碼復(fù)雜。網(wǎng)絡(luò)通信136-2 Line Configuration1.Topology(1) Topology: The topology of a data link refers to the physical arrangement of stations on a transmission medium. If there are only two stations, the link is point-to-point. If there are more than t

18、wo stations, then it is multipoint topology. Traditionally, a multipoint link has been used in the case of a computer (primary station) and a set of terminals (secondary stations). (2)Half Duplex: With half duplex transmission, only one of two stations on a point-to-point link may transmit at a time

19、. (3)Half Duplex: Half duplex is also referred to as two-way alternate, suggestive of the fact that two stations must alternate in transmitting. This can be compared to a one-lane（單車道）, two-way（雙向）bridge.網(wǎng)絡(luò)通信14Examples Point-to-point MultipointHostTerminalTerminalTerminal(Primary)(Secondary)PrimaryS

20、econdary網(wǎng)絡(luò)通信156-3 InterfacingWhy Most digital data processing devices have limited data transmission capability. （例如：只能產(chǎn)生NRZ-L這樣的數(shù)據(jù)；且傳輸距離有限） Consequently, it is rare for such a device (terminal, computer) to attach directly to a transmission or networking facility.DTE and DCE The devices, including

21、terminals and computers, are referred to as data terminal equipment (DTE). A DTE makes use of the transmission system through the mediation (仲裁) of data circuit-terminating equipment (DCE). An example of DCE is a modem. 網(wǎng)絡(luò)通信16Examples網(wǎng)絡(luò)通信17 DCE的用途的用途 The DCE is responsible for transmitting and recei

22、ving bit, one at a time, over a transmission medium or network. The DCE must interact with the DTE. In general, this requires both data and control information to be exchanged. This is done over a set of wires referred to as interchange circuits. The two DCEs in both sides need: Understand each othe

23、r; With the same coding scheme; With the same data rate;網(wǎng)絡(luò)通信18 DCE的主要特性的主要特性（With Four important characteristics） Mechanical: actual physical connection of the DTE to the DCE（包括插頭、插座、導(dǎo)線等）. Electrical: with the voltage level and timing of the voltage changes（使用相同的編碼和電平來表示相同的事情，且信號持續(xù)時(shí)間相同，這些特性可確定數(shù)據(jù)率和傳輸

24、距離）. Functional: specify the functions that are performed by assigning meanings to each of the interchange circuits（功能可分為數(shù)據(jù)的、控制的、時(shí)序的和電氣的等）. Procedural: specify the sequence of events for transmitting data, based on the functional characteristics of the interface.網(wǎng)絡(luò)通信19V.24/EIA-232-F InterfaceV.24是應(yīng)用

25、最廣泛的ITU-T標(biāo)準(zhǔn)；V.24僅規(guī)定功能（functional）和過程（procedural）方面的接口；V.24的電氣（electrical ）和機(jī)械（mechanical）方面的接口參考其它標(biāo)準(zhǔn)；EIA-232-F與V.24的對應(yīng)關(guān)系： 機(jī)械規(guī)約：ISO2110 電氣規(guī)約：V.28 功能規(guī)約：V.24 過程規(guī)約：V.24EIA-232最早由美國電子工業(yè)協(xié)會(huì)于1962年發(fā)布，又稱RS-232。目前的第7版于1997年發(fā)布。網(wǎng)絡(luò)通信20V.24/EIA-232引腳規(guī)定引腳規(guī)定網(wǎng)絡(luò)通信21機(jī)械特性 25芯插頭，由ISO定義； 用于連接DTE與DCE（但許多實(shí)際應(yīng)用中所使用的交換電路遠(yuǎn)少于25線

26、）電氣特性 定義DTE與DCE之間的傳輸信號； 均使用數(shù)字信號（二進(jìn)制的數(shù)據(jù)）； 以公共地為基準(zhǔn)，+3V為邏輯0（即NRZ-L編碼）; 信號速率20kbps; 傳輸距離15m功能特性 25芯分為數(shù)據(jù)、控制、時(shí)序及接地等幾類； 每個(gè)方向上有一條數(shù)據(jù)線（Full duplex）； 兩條輔助數(shù)據(jù)線路，半雙工時(shí)使用； 控制線路16條； 環(huán)回測試（loopback test）：is a useful fault isolation tool. 網(wǎng)絡(luò)通信22環(huán)回測試環(huán)回測試: Suppose that a user at a personal computer is communicating with a server by means of a modem connection and communication suddenly ceases. The problem could be with the local modem, the communication facility, the remote modem, or the remote server. A network manager can use loopback tests to isolate the fault. Local loopback checks the functioning of t